Relationship between reactive group chemistry and printing properties of heterofunctional reactive dyes via screen printing

Screen printing of cotton fabric using newly synthesized azo reactive dyes was carried out in the present study. Functional group chemistry and its effect on the printing properties of cotton fabric by varying the nature, number and position of reactive groups of synthesized azo reactive dyes (D1–D6) was studied. Different printing parameters (Temperature, alkali and urea) and their effect was explored on the physicochemical printing properties e.g., fixation, color yield, and penetration of the dyed cotton fabric. Data revealed that dyes with more reactive groups and having linear and planar structures (D-6) showed enhanced printing properties. Spectraflash spectrophotometer was used to evaluate the colorimetric properties of screen-printed cotton fabric and results showed superb color buildup. Printed cotton samples displayed excellent to very good ultraviolet protection factor (UPF). Presence of sulphonate groups and excellent fastness properties may entitle these reactive dyes as commercially viable for urea free printing of cotton fabric.


Synthesis of heterofunctional reactive dyes. Condensation, diazotization of sulphatoethylsulphone
and H-acid coupling reactions were carried out for the synthesis of heterofunctional reactive dyes (D-1 to D-6) as described in the previous study 3 . The structures of the synthesized dyes are given in Scheme 1. New dye structures were characterized and confirmed through ultraviolet-visible, Fourier transform infrared and electrospray www.nature.com/scientificreports/ ionized mass spectrometry analytical techniques as discussed in previous study 13 . Characterization results are also given in supplementary data. Evaluation of fixation, penetration and colorimetric data of printed samples. K/S values of the printed cotton samples were determined before and after washing treatments which reflects the fixation percentage of dye covalently attached to the fabric. Spectraflash spectrometer was used for the determination of the color strength of printed samples at their particular λ max . Calculations of fixation ratio were carried out using Eq. (1) 25 .
where (K/S) 1 and (K/S) 2 denotes the color strengths of the printed cotton samples before and after washing, respectively. The penetration percent (P %) was calculated according to Eq. (2).
where (K/S) b and (K/S) f represent the non-printed back face and printed front face values of K/S of fabric, respectively. If penetration percentage higher than it shows the greater penetration of the dye into the fabric 26 .
Printed samples were subjected to LCH and CIELAB system for the evaluation of colorimetric data L*, a*, b*, C* and h* representing lightness/darkness, redder/greener, redder/greener, chroma, and hue, respectively for one color determination 27 . Spectraflash spectrophotometer 7000 A (Color Eye, Gretagmacbeth) was used

Results and discussion
Effect of temperature on printing properties of reactive dyes. Reactive dyes first adsorbed on the fabric surface through an ionic bond. Once assembled, the ionic bond was then converted to a covalent bond at required optimum temperatures 28,29 . The optimum temperature for steaming of printed cotton samples was 105 °C at which maximum penetration, fixation, and color yield was obtained 30 as shown in Fig. 1. Rate of dye fixation decreased at high temperature because activation energy for dye-fiber interaction is 9.2-15.8 kcal while for dye-water interaction is 16.4-26.2 kcal 28 . So, at higher temperature than optimum the dye-water reaction rate increased which is responsible for dye hydrolysis and in turn poor washing fastness. Dye hydrolysis also increases the loss of dye in the effluent which caused potential pollution problems. At low temperature dye molecules mobility could be low which required more printing time. Klančnik 32 also studied the effect of temperature on the dyeing behaviour of the reactive dyes 31,32 .

Urea influence on the fixation and color yield of reactive dyes.
The key function of the urea in the printing paste is to facilitate the dye-fiber interaction by reducing the dye aggregation and increasing its solubility in the reaction medium 32 . It also retards the evaporation of water during drying and swelling of cellulosic fabric in the printing process 33,34 . All heterofunctional reactive dyes used during the printing technique were watersoluble, vinylsulphone based, and have sulfonate groups which increased their solubility in printing paste 29,35 . So, color yield and fixation % of the printing samples were not affected greatly by the urea concentration. The effect of urea on the K/S and fixation values of dyes (D-1 to D-6) is shown in Fig. 2. These results demonstrated that urea free dyeing is possible for vinylsulphone based dyes having sulfonate groups in their structures 26,36,37 . Urea poses many environmental and ecological issues due to high nitrogen contents in the printing effluent.
Alkali influence on the fixation and color yield of reactive dyes. Alkali was very important for screen printing of dyes because sulphatoethylsulphone reactive groups were converted into their active vinyl-  www.nature.com/scientificreports/ sulphone form through addition reaction which was possible in the presence of alkali and at appropriate pH. Alkali was also necessary for the activation of the cotton material so that fabric pores are in exact orientation in the printing medium for covalent bonding with the dye molecules 38,39 . The effect of alkali on the color yield and % fixation of the printed cotton samples is shown in Fig. 3. It was clear from data that printing properties (K/S and fixation, penetration) were increased with the alkali concentration up to a specific limit. Beyond this limit, printing properties started decreasing because the rate of dye hydrolysis might be increased in the printing medium 40,41 .
Colorimetric data. Colorimetric properties of the printed cotton fabric with heterofunctional reactive dyes were obtained through spectraflash spectrophotometer and results are presented in Table 1. Different patterns of screen printing were employed on cotton fabric by six heterofunctional azo reactive dyes which are represented by UPF and color fastness of the printed cotton samples. UPF factor of the control (non-printed) fabric was 3.5. Results from Table 2 showed that the UPF factor of the printed fabrics was excellent to very good at 3% dye shade. UPF value increased as dye fixation and penetration increased on the cotton fabric and results showed that D-6 has highest UPF value having highest color yield 35,42 . This is related to its more planar and sterically less hindered trifunctional structure responsible for highest penetration and fixation of dye on the cotton fabric as reactive groups are present at the para positions in D-6. D-2 has lowest UPF value which may be attributed to its bifunctionality and meta position of the reactive groups.  www.nature.com/scientificreports/ Data outcomes from Table 2 showed good (4) washing fastness of printed cotton samples which may be credited to dye molecules fixation through covalent bond formation with the cotton samples 43 . Crock fastness was also good of the printed cotton samples. Bifunctional reactive dyes have moderate rubbing as compared to trifunctional reactive dyes. Hydrolyzed dye on the fiber surface was responsible for staining during rubbing fastness and results from Table 2 showed that contents of hydrolyzed dye was minimum because maximum dye molecules fixed on printed samples dye to strong covalent bonding. Good to excellent (4-5) light fastness was observed from results which may be attributed to the fact that H-acid azo dyes chromophores can undergo to azo-hydrazone tautomerism making H-acid azo dyes stable to photoreduction 44 . Presence of heterofunctional reactive groups (triazine and vinylsulphone) in newly synthesized azo dyes makes them stable to both alkaline and acidic conditions showing good perspiration fastness 36 . Fastness to chlorinated water showed moderate (3) results for bifunctional dyes while moderate to good (3-4) results were obtained for the trifunctional reactive dyes.
Reactive group chemistry and printing properties. There is an important role of functional group chemistry and dye structure in the proper dye fiber linkage. The heterofunctional reactive groups are responsible for greater affinity of dye molecule towards cellulosic fiber. Coplanar, linear, sterically less hindered structures having more functional groups are key factors in attaining good quality printing with excellent color yield. In this context, in present study printing properties were investigated by changing the number and positions of  www.nature.com/scientificreports/ reactive groups during screen printing of cellulosic fabric 45 . Reactive dyes attached and fixed on the cotton fabric by forming a permanent covalent bond between the fiber hydroxyl and dye reactive groups [46][47][48] . As the reactive group's number increased, the extent of bonding increased which ultimately increased fixation and color yield. The increasing order of printing properties D6 ˃ D-5 ˃ D-4 ˃ D-3 ˃ D-2 ˃ D-1 showed that trifunctional reactive dyes revealed enhanced printing properties (penetration, fixation, UPF value and color yield) as compared to bifunctional reactive dyes (Table 3). Dye structures should be more planar and sterically less hindered so that effective dye fiber interaction takes place, which consequently increased the dye penetration and fixation in the cellulosic fabric pores 3,46,49 . As penetration and fixation increased, the color yield of printed fabric automatically increased 50,51 . D-6 having more planar and sterically less hindered structure (reactive groups present at para positions) had increased color yield, higher fixation, penetration and UPF value as compared to D-1, D-2, D-3, D-4, and D-5. The penetration and fixation of dyes increased into the cotton fabric by increasing the number of reactive groups because more reactive sites were available to fiber substrate for binding which leads to high color yield [52][53][54] . Therefore, trifunctional reactive dyes have high UPF value than bifunctional reactive dyes as UPF value depends on color strength 27,43 . Synthesized dyes were vinylsulphone based having sulfonate groups in their structures so they have a higher solubility in water and consequently in the printing paste which makes them more suitable for urea free printing 26 . Different patterns of screen-printed cotton fabrics by heterofunctional reactive dyes are shown in Fig. 4.

Conclusion
The current study showed that reactive group chemistry significantly influences the printing properties of cotton fabric. The optimum temperature for steaming of printed cotton samples was 105 °C which prominently affects the printing properties of reactive dyes. Six heterofunctional reactive dyes were sensitive to alkali but urea concentration had no noticeable effect on the fixation and K/S values. All dyes showed excellent to good fastness properties onto the printed cellulosic fabric which may be attributed to the permanent covalent bonding of reactive dyes. Trifunctional reactive dyes (D-3 to D-6) have enhanced printing properties as compared to bifunctional reactive dyes (D-1 and D-2). The penetration and fixation of dyes increased into the cotton fabric as the number of reactive groups and planarity increased and steric hindrance decreased in the dye structures which consequently increased the K/S value. UPF of the printed samples was excellent to very good. Vinylsulphone reactive dyes having sulfonate groups in their structure have high solubility in printing paste. Therefore, it is suggested that these six reactive dyes could be commercially practicable for urea free printing of cellulosic fabric. www.nature.com/scientificreports/